Strippable layer relief imaging process

ABSTRACT

Thermoplastic deformation images are produced by xerographically deforming softenable films temporarily overcoated on conventional xerographic sensitive members. By the use of appropriate support layers for the softenable films, the deformed films can be readily separated from the sensitive member while preserving the image.

United States Patent I inventors App]. No. Filed Patented AssigneeSTRIPPABLE LAYER RELIEF IMAGING PROCESS 6 Claims, 1 1 Drawing Figs.

US. Cl. 96/1.l, 96/].5,117/201,117/218,178/6.6TP, 340/173 TP, 346/74 TP,355/9 Int. Cl. 841m 5/20 FieldofSearc 96/l,1.l,

1.5 536/155 15', i 55112513754 PC, 74 TP; 178/6.6 TP, 6.6 R; 117/201,218; 355/9 [56] References Cited UNITED STATES PATENTS 2,833,648 5/1968Walkup 96/1 2,904,431 9/1929 Moncreff-Yeates.. 96/1 2,975,052 3/1961Fotland 96/1 3,052,006 9/1962 Dreyfoos et ai. 346/74 3,063,872 11/1962Baldebuck 117/211 3,095,324 6/1963 Cusano 1 17/215 PrimaryExaminer-Charles E. Van Horn Attorneys-Frank A. Steinhilper and StanleyZ. Cole ABSTRACT: Thermoplastic deformation images are produced byxerographicaily deforming softenabie films temporarily overcoated onconventional xerographic sensitive members. By the use of appropriatesupport layers for the softenable films, the deformed films can bereadily separated from the sensitive member while preserving the image.

PATENTEnnm 26197! 3, 1 5,3 7 sum 1 BF 2 ,0 A INVENTOR F/G. 5 LESTERCORRSIN JOAN R. EWING A TTORNE) This invention relates to electrostaticprinting and, in particular, to forms of electrostatic printing in whichthe latent electrostatic image is made visible by the deformation of acompliant layer. in xerography. as it was taught for example by Carlsonin US. Pat. No. 2.297.691. as insulating photoconductive layer wassensitized by charging to an electrostatic otential and then the latentel'ectrostaticimage was formed by exposing the layer to an image patternof light and shadow to selectively dissipate the electrostatic charge.The latent electrostatic image thus formed has been conventionallydeveloped by means of an eiectroscopic pigmented powder. The powderimage then must be fixed to a second layer or transfer sheet in order toprevent disturbance of the powder image. These steps of development andfixing of the image are time consuming and require considerablecomplexity in the apparatus. More recently, attempts have been made todevelop latent electrostatic images by deformation of compliant layersas produced by the electrostatic forces of the image. This eliminatesthe necessity of a developer material. reduces the development time, andthe complexity of the equipment. However. conventional xerographicmaterials and methods have not beenfound to lend themselves readily tothis type of deformation imaging and attempts to malte use of the moreobvious methods have produced weak and impermanent images. Attempts toprovide adequate deformation images have led to systems of increasingcomplexity. For example. systems operating in a vacuum and systems usinga deformable liquid with a further development or transfer step torender it permanent. in some instances, it is particularly desirable toproduce high resolution images so that large quantities of image datamay be stored in a relatively small space or on a relatively smallamount of recording material. Thus. for example. when recordingequipment is used in different types of missiles and space vehicles. itis desirable that the amount of recording material needed to store agiven amount of information be relatively small and that the equipmentnecessary to produce the image be likewise small without unnecessaryoperative stages. The necessity in con ventionalxerography of a bulkydevelopment stage and of relatively high heat fixing with its attendanthigh power consumption has ruled it out in the past for purposes of thisnature. Y

Now in accordance with the present invention. it has been discoveredthat deformation images can be produced by xerographically deformingsoftenable films temporarily overcoated on conventional xerog raphicsensitive members. it has further been discovered that the use ofappropriate support layers for said films enables the deformed films tobe readily separated from the sensitive member while preserving theimage. Thus it is an oblect of the invention to define self-supportingdeformable overlayers for xerographic imaging.

It is an additional object of the invention to define methods fordeformation printing using a photoconductive insulating layer coatedwith a separable defonnsble member.

it is an additional object of the present invention to define apparatusfor xerographlcally deforming a separable member.

Further objects and features of the invention will become apparent whilereading the following description in connection with the drawingswherein:

FIG. I is a.dlagrammatic illustration of charging a thermoplastic coatedserographlc plate;

FIG. 2 is a diagrammatic illustration of exposing a sensitizedthermoplastic coated xerographic plate;

F IG. 3 is a diagrammatic illustration of a second method of exposing asensitized thermoplastic coated xerographic plate;

H0. 4 is a diagrammatic illustration of a second charging step employedin accordance "with an embodiment of the present invention;

HO. 5 is a diagrammatic illustration of simultaneous charging andexposing of a thermoplastic coated xerographic plate;

FIG. 6 is a diagrammatic illustration of vapor development of adeformation image;

HO. 7 is a diagrammatic illustration of heat development of adeformation image;

FIG. 8 is a further embodiment of heat development of a deformationimage;

FIG. 9 is a diagrammatic illustration of simultaneous expo sure anddevelopment of a thermoplastic coated xerographic plate;

H0. 10 is a diagrammatic illustration of an embodiment using a coloredthermoplastic layer in accordance with the present invention; and.

FIG. ii is a diagrammatic illustration of apparatus for formingdeformation images on a separable thennoplastic layer.

Some thermoplastic materials have been found to deform readily whensoftened while under the influence of a latent electrostatic image. Anassembly of a xerographic plate carrying a layer of such a thermoplasticmaterial is illustrated in FIG. 1. This arrangement is adapted inaccordance with the invention to sustain either voltage gradients orelectrostatic charge density gradients on a surface which is thendeforma ble in accordance with such gradients. The plate is shown ascomprising conductive substrate l0 coated with photoconductiveinsulating layer it as is conventional. Over the photoconductiveinsulating layer is interlayer 12 which is. in turn. coated withcompliant thermoplastic l3. Substrate 10 may be any conventionalconductive baclting as used in conventional serography. Thus. it may bebrass. aluminum. or other metal or it may be a flexible conductivematerial such as conductive paper or a plastic material coated with aconductive coating such as tin oxide or copper iodide or it may be atransparent material such as glass or clear plastic with a conductivecoating of tin oxide. copper iodide. or the like for transparency. Anyconventional photoconductive insulator such as vitreous selenium.anthracene. sulfur. zinc oxide in a binder material. or otherphotoconductors may be used in insulating binders.

'However, as will be disclosed below, photoconductors adapted to forminguniform homogeneous layers have been found preferable for highresolution purposes. interlayer 12 serves as a barrier layer between thethermoplastic and the photoconductive insulating layer and also servesother important functions. it protects the photoconductor from anyinteraction with the particular thermoplastic used. it serves as 7 anisolation layer during development to protect the photoconductor fromthe effects of the solvent vapor or the effects of the heat and at thesame time. helps to maintain electrical insulation between thethermoplastic layer and the photoconductive layer. A further function ofinterlayer i2 is in separable deformation layers in which case theinterlayer serves as a separation support. This is essential sincesuitable compliant layers such as the various insulating thermoplasticshave inadequate dimensional stability as self-supporting layers tomaintain an undistorted image during separation. Since somephotoconductive materials such as many of the organic photoconductorsshow no deleterious reaction to most thermoplastic materials or totemperatures used for softening such materials. the use of interlayerswith them serves no purpose unless separation is required. Many of thehigh melting point plastics are suitable for use as interlayer 12. Theyare preferably tough. electrically insulating. and highly transparent.High dimensional stability is required where used for separable layers.in some embodiments of the invention. as will be seen below. however.the interlayer need not be transparent. One preferred material isVinylite" (trademark of Carbide and Carbon Chemical Company. New York.NY.) polyvinyl chloride. This has been found preferably because of itshigh insulating qualities. low reactive effects. high tensile strength.and a softening point above the temperatures necessary for deforming lowmelting point thermoplastic materials as found suitable for use with thepresent invention. Also suitable for interlayer 12 are other polyvinylchloride or polyvinyl acetate resins. or mixtures thereof, as well aspolyethylene terephthalate and other plastics having the desiredcharacteristics set forth above. Thermoplastic layer 13. in accordancewith the present invention. must be adequately insulating to support anelectrostatic charge on its surface and is preferably selected to becapable of maintaining such a charge while it is softened by heat orvapor to a point where deformation can occur. it is further preferablethat the thermoplastic have a low softening temperature so that it willbe deformed from the effects of a latent electrostatic image attemperatures below about 140' F. it is further desirable that thethermoplastic be free from flow effects at normal room temperatures.that is, below about 90' F. A preferred material has been found to be"Staybelite" (trademark of Hercules Powder Company. Wilmington Del.)Eater No. l0. This material has been found preferable due to longer termstorage characteristics for preservingthe image than has been found inother thermoplastica having similar electrical resistance and softeningtemperatures. Other suitable materials are Piccolastic" (trademark ofPennsylvania industrial Chemical Corporation, Clairton. Pa). Type A withmelting point from 50'-7$ C.; "Neviliac" soft (trademark of NevilleCompany. Pittsburgh. Pa); and other transparent thermoplastic resinshaving amelting point generally between 40' and 80" C. and electricalresistivity of at least ohm-centimeters at 30 C. The thermoplastic layerand interlayer are preferably itept thin for high resolution and in thecase where the layers are ermanently bonded. the interlayer may be asthin as onetenth of a micron. Where separable layers are used. theinterlayer must be thick enough to provide the necessary strength anddimensional stability for separation. Thus. for separable layersinterlayer 12 may vary between a few microns and about i mil dependingon the strength of the material used. The thinner layers may be appliedto the photoconductive insulating layer by permanently bonding in a dip.spray. or whirlcoating procedure or by vacuum evaporation. For dip.spray or whirl-coating the plastic is dissolved in a solvent and appliedto the photoconductive layer in a liquid form and then hardened byevaporation of the solvent. The thermoplastic layer may be coated overthe interlayer in a similar manner. Where separable layers are used. theinterlayer is preferably in the form of a self'supporting web which iscoated with the thermoplastic layer by one of the procedures suggestedabove. The process steps to form the image reproduction in accordancewith the invention are capable of various manipulations which aregenerally selected in accordance with the particular conditions anddesired results. FIG. I shows a conventional preliminary charging stepthat may be used to sensitize the thermoplastic coated plate of theinvention. Corona charging device 15 connected to potential source 16 isarranged to apply a voltage of betweenapproairnstely lOO and L000 voltsto the surface of thermoplastic layer 13. While either positive ornegative charging may be used. positive charging is illustrated asindicated by the plus signs shown at the surface of the thermoplasticwith matching negative charges shown by minus signs in the substrate 10.

FIG. 2 illustrates exposure to an image pattern of light and shadow. Thethermoplastic layer need not be transparent in which case. exposure ismade through substrate l0. Substrate l0 in FIG. 2 is illustrated as atransparent glass or plastic layer with transparent conductive coating17 to enable exposure of the serographic plate through the bacit. Thistype of exposure has the advantage in the present invention in that theinterlayer l2 and the thermoplastic layer 13 may have poor opticalqualities and may be colored to the extent of being opaque if desired.it has been found generally preferable to obtain opacity of the plasticcoated side ofths plate by coloring interlayer 12. Thus, interlayer 12may be colored by nigrosine dye. for example. which will produceadequate opacity in s 10- micron layer of polyvinyl chloride if added inthe proportion of about 10 to 20 percent weight by volume of nigroslneto plastic. Addition of most colorants in sufficient strength to produceopacity in the deformable layer has generally been found to reduce thebulk resistivity to an excessive degree. if the thermoplastic layer andthe interlayer are opaque. the development step is simplified as will beseen below. in FIG. 2.

xerographic plate. The crosshatched section 21 of the rojccted imageindicates a dark section with little or no illumination while theuncrosahatchcd section of the projected image 22 is a light or highillumination portion of the image. Where illumination reaches thephotoconductive layer ii, the resistance of the layer decreases so thatnegative charges in thc substrate pass up through the photoconductor tothe interface between the photoconductor and interlayer 12. Where thephotoconductor is illuminated. the electrical capacity between thesurfaces bearing the opposite electrical charges t increased due to thedecrease in spacing between the Charge. carrying surfaces. increasingthe capacity in this way without changing the charge quantity decreasesthe voltage of the charged surface in accordance with the formula Q-CE.0 represents the quantity of electric charge in couiombs. C equalscapacity in farads, and E represents voltage. it. will be seen that whenthe capacity (C) is increased while the charge quantity (Q) ismaintained constant. that the voltage (E) will be reduced. Thus, themeasurablepotential on the surface of the thermoplastic becomes lessover the illuminated areas. than over the darlt areas.

FIG. 3 is an alternative embodiment of the exposure step in which theimage pattern of light and shadow is projected onto the photoconduct orthrough the thermoplastic layer. As is obvious. this requires a highdegree of transparency in the thcrmoplastic layer and in any interlayerthat exists. After exposure. the image may be developed immediately orthe voltage differentials existing on the surface of the thermoplasticlayer can first be changed to variations in charge density.

H0. 4 illustrates a procedure for changing the voltage gradients intovariations in charge density. This is done by repeating the chargingstep as performed in the first sensitization of the plate. Since thecharging devices conventionally used in xerographic processes arevoltage responsive. the charging device sees the reduced voltage overthe illuminated areas and applies more charge as indicated by the doublerow of plus signs over the previously exposed areas of the plate. in theareas where the plate was dark during exposure. the charging device seesthe original voltage and applies no additional charge. Thus, the chargequantity is increased only in the areas that were illuminated during theexposure step. There is a significant difference between the forcespresent after a second charging as in FIG. 4 compared with those presentimmediately after the exposure step. With just the voltage gradients onthe surface. only an edge effect image can be produced while after thesecond charging, it is possible to produce effects on larger areas. Thiswill be described in more detail in connection with image developmentillustrated in FIGS. 7-10.

It is possible to simultaneously charge and expose a then moplasticcoated aerographic plate as illustrated in FIG. 5. This produces thesame effect as shown in FIG. 4 to a pronounced degree. Thus. since theexposure is going on continuously during charging. charges of onepolarity in the substrate may continuously drift up through thephotoconductivc layer in the illuminated areas permitting increasedcharging in I the respective thermoplastic surface areas. This permitsgreater relative charge density in the illuminated areas as compared toprocesses described in connection with FIG. 4 in which the conductivityof the photoconductor is shut off during the second charging. While inFIG. 5. the image is illustrated as projected from the same side of thecoated aerographic plate as that on which the charge is applied. it is.of course, possible to project the image through a transparent substratein the manner of FIG. 2 while simultaneously charging the surface of thethermoplastic layer.

Deformation of the thermoplastic layer in the image pattern can beproduced by two general methods. One is to soften it by heating and theother is to apply a solvent preferably in a vapor form to soften thelayer. Heat is considered preferable since it is more readily controlledand its action can be stopped more rapidly than that of the solvent.Following expoan image I! is projected through an optical system 20 ontothe sure as in FIGS. 2 and 3, deformation development must bc performedwith the photoconductor shielded from light. If exposure has beenmadethrough a transparent substrate and an opaque plastic layer. shieldsthe photoconductor on the side of the deformable layer, as has beensuggested above, thermoplastic layer 13 may be developed by heat orvapor while under illumination. Also where recharging has producedcharge density variations on the deformable surface, development may becarried out under normal illumination.

FIG. 6 illustrates the use of the solvent vapor..The plate carrying thethermoplastic layer can be passed into chamber 25 containing a solventvapor for the thermoplastic. With a thermoplastic layer of "Staybelite,"suitable solvents are ethylene dichloride, carbon tetrachloride, hexane,trichloroethylene, or the like.

FIGS. 7 and 8 show development by means of heat. The heat source in FIG.7 is indicated as an infrared lamp 26 and the heat source in FIG. 8 isillustrated as an electrical resistance heating element 27. The infraredheat source is particularly suitable when one of the plastic layers iscolored and exposure is made through a transparent substrate. Thecoloring absorbs the infrared radiation giving preferential heating.Accordingly, interlayer 14 in FIG. 7 is illustrated as an opaque layer.

It is also possible to develop an image by softening the thermoplasticlayer during the exposure step. This is illustrated in FIG. 9 in whichexposure from image 18 is made through transparent substrate while anelectrical resistance heating element 27 applies softening heat to thesurface of the thermoplastic layer.

The amount of heat orsolvent to be applied will depend upon thecharacteristics of the thermoplastic layer and of thickness."Staybelite, by way of example, should generally be heated to a surfacetemperature of about 45-70 C. In any case, the viscosity of the materialshould be reduced to between about l0 to ID poises. A viscosity belowthis range generally produces a loss of surface charge which may be dueto mobility of ions in the material as it becomes more fluid. Aviscosity above this range will still permit deformation, however thetime required will run into several seconds or even minutes which isgenerally excessive for practical use. It should also be noted in thisconnection that repeated heating of vitreous selenium to temperaturesabove 50 C. will lower its electrical resistance. However, with otherphotoconductors, such as the organic photoconductors, the repeated useof high temperatures has no significant effect on electricalcharacteristics. In at least one embodiment of the invention, a lowerelectrical resistance in selenium is not necessarily harmful as will beseen below.

In a particularly compact embodiment of the invention, the process stepsof charging, exposure and development are carried out simultaneously asillustrated in FIG. 10. A further discussion of this embodiment is givenin connection with techniques for enhancing image visibility.

After the material has been exposed as illustrated in FIG. 2 or 3 andthen developed as illustrated in FIGS. 7 and 8, or if it issimultaneously exposed and developed as illustrated in FIG. 9,deformation can take place in accordance with the following theory whichis presented by way of explanation but not intended to be limiting:

After electrostatic charging and before exposure, large fields exist inboth the overcoating and the photoconductor in amounts inverselyproportional to the dielectric constant. That is,

and

where E is the field, Q/A the charge per unit area, K the dielectricconstant, :1 the layer thickness, and ph and lit the subscripts for thephotoconductive and thermoplastic layers.

For typical xerographic use, the potential across a 20- 'micron seleniumplate is about 600 volts, so that f'Z'if2 3;99:.99l9Ff/ i and across thethermoplastic with about one-third the dielectric constant,

E r-900,000 volts/cm. After exposure, the field in the photoconductorwould be reduced to a value proportional to the induced charge remainingon the substrate, so that a fully exposed area will have zero fieldwithin it. On the other hand, the field across the thermoplastic doesnot change (in large uniform areas). What does change is the potential.The potential of the free surface is given by V,,,,,,,.,==41rmd,,,+41m,,, d where (7030., the initial charge and 0-,, charge remainingon the substrate after exposure. If now the plastic is softened, nothingwill happen in large ex posed areas, because there has been no change inelectrostatic stress. However, at the boundary between a region ofhigher potential (unexposed) and lower potential (exposed) an additionalelectrostatic field will be generated on both sides of the edge.

This will create additional electrical and mechanical stress at theexposed edge and reduced stress on the dark sideof the edge, to givedeformation in the softened film as shown, for example, in FIG. 7.

As part of an extensive computer analysis of fields above electrostaticsurfaces, a calculation yields a value of 6X10 volts/meter for thenormal components of the field at an edge between charged and dischargedportions of the plate. For such a field and a charge density of l.4Xl0"coulombs/crn', the deforming pressure is P=6X 10 x 1 .4X 1 0=800newtons/m'=8000 dynes/cm. For a line electrostatic image l.0 cm. longand 0.1 cm wide, this yields a force of dynes.

It should be noted that when a simultaneous development and exposure isused as in FIG. 9, a slightly enhanced image is produced since the firstdisplacement of the surface during development produces additionalvariations in the layer capacity at the image edge increasing thecontrast effected by the exposure and thus permitting a greaterdeformation.

As implied by the above theory of operation, in FIGS. 7, 8 and 9 asillustrated, an edge deformation of the image occurs at the position ofthe potential gradients 28. While this method will not reproduce solidareas, this edge effect type of image is capable of very high resolutionand can be readily projected by the use of Schlieren optics or the like.

Where solid area reproduction is desired, a modification of thereproduction process has been found to permit limited solid areadeformation. An example of this modification is the second charging stepas illustrated in FIG. 4, or in a simultaneous charge and expose methodas in FIG. 5. Thus, if the exposed material is recharged to bring it touniform potential, the field produced by the charge density is increasedin the exposed area. The image response of the softened plastic isgenerally to depress and create large thinner areas whose surfaces areparallel to the original surface. The image on such a layer yields phasedifferences which can be observed by a phase contrast method, howeverthe ability of the material to squeezed out of an area by theimage-dependent electrostatic force is greatly influenced by theconditions in the surround ing areas and accordingly this method is mostuseful where the areas to be depressed are relatively small. Inreproducing continuous tones or large solid areas, a screening processis preferred to break the large solid image areas into readily deformedsmall areas.

With increased charge density in the exposed areas, a solid areadeformation can be produced as indicated by the depressed areas 30 inFIG. 6. While development of the solid area deformation is illustratedin FIG. 6 by solvent vapor and while the edge deformation developmenthas been illustrated in FIGS. 7, 8 and 9 by heat, it is completely amatter of choice which form of development is used for either the solidarea deformation or the edge deformation. As has been previously stated,heat development is generally preferable in both instances since it ismore readily controlled.

The solid area deformation produced by differences in charge densityproduii's an image of plane parallel areas at different levels. Thistype of an image is not readily observable and requires aphase-sensitive imaging system for display purposes. Several techniquesfor enhancing visibility of the deformed image have been found, however,that permit ready observation of such an image. FIG. 10 shows an exampleof this in which deformable thermoplastic coating 32 is of contrastingcolor or of highly differentiated color density relative to interlayer31. Thus, for example, layer 31 may be transparent while layer 32 iscolored as by the addition of a small amount of nigrosine. These layerscan be readily applied to the plate by dip coating steps in which layer31 is permitted to harden and dry before the application of layer 32.Upon forming and developing a solid area image of difl'erent chargedensities, the exposed areas of the uppermost layer 32 are depressed andthus thinned out to the point where it is virtually invisible and thelower layer 31 is exposed to observation. This produces an immediateviewable image. It is also possible with separable layers to obtain atransparency. The deformable thermoplastic layer colored by somecolorant such as nigrosine dye is coated on aseparable interlayer thatis highly transparent. After image formation and development, thedepressed areas of the thermoplastic layer being relatively thin containrelatively less dye and transmit more light than the areas that are notdepressed. Accordingly, the interlayer can be stripped off the platecarrying the deformed, dyed, thermoplastic layer and utilized in aconventional projector. Due to the effect of the usual colorants inlowering the resistivity of the thermoplastic it has been founddesirable when using dyed deformable layers to charge, expose anddevelop simultaneously. Since this requires minimum storage time for theelectrostatic charges on the deformable surface, a substantially lowerbulk resistivity is compatible. With this simultaneous processing,resistivities as low as l" ohm-cm in the deformable layer have stillpermitted image deformation. The illustrated embodiment, FIG. 10, isarranged to provide exposure through substrate while charging anddeveloping from the opposite side of the layered assembly. While thisembodiment has been chosen for ease of illustration, it is just assuitable to use an opaque substrate and expose, charge and developsimultaneously from the side facing the deformable surface. Substrate 10and photoconductive layer 11 are the same as described in previouslydisclosed embodiments. interlayer 32 is preferably a clear plastic andlayer 31 is a thermoplastic having a lower softening temperature thanlayer 31. For example, layer 32 can be polyvinyl chloride and layer 31can be Piccolastic" A-75. Layer 31 contains a dye such as nigrosine.Effective coloring in a S-micron layer of thermoplastic is provided byabout 10 percent by weight of nigrosine base per volume of thermoplastic(CGS units.) Thinner layers require higher percentages of nigrosine andthicker layers require lower percentages of nigrosine to obtain the samemaximum image density. ff

Heating elements 33 are shown in association with charging device 15. Asthe charging device is operated to apply an electrostatic charge, theheating elements function to heat the same area to the deformationtemperature of deformable layer 32. Source of illumination 34 isoperative in conjunction with optical system 20 to project a light andshadow pattern of image subject 18 onto photoconductive layer 11.Voltage source 29 applies operating potentials to charging device 15,heating elements 33, and source of illumination 34 simultaneously by aganged switch 39. This simultaneous method has been found to be fast andis adapted to compact systems.

A method that avoids the use of colored layers requires an extradevelopment step. By this method, a depressed area image is formed byany of the processes previously discussed and then a high viscosity orpastelike pigmented material is wiped over the surface of the deformedplastic so that it fills in the depressions. Pigmented materials thathave been found useful for this purpose include printers ink and many ofthe graphite dispersions sold under the trademark Dag" such as Aquadagby Acheson Colloids Corporation of Port Huron, Mich.

A reusable temporary overcoating system is illustrated in FIG. 11. Thisfigure shows the continuously operable apparatus for producing deformedthermoplastic images on a thermoplastic layer overlying a continuousphotoconductor web. The photoconductive web 35 comprises aphotoconductive insulating layer on a conductive backing material whichis carried onto rotatable cylinders 36. Cylinders 36 are connected forrotation to a drive means 49. Arranged in sequence in the direction ofrotation of the photoconductive web is erasing station 37, chargingstation 38, exposure station 40, recharging station 41, developmentstation 42 and separating station 43. The thermoplastic layer 45 coatedon a heat resistance transparent plastic support member 46 is fedthrough the erasing station 37 and into traveling contact with thephotoconductive web by feed means 44. The surface of photoconductive web35 is precharged at electrostatic charging station 38 before contactingplastic support member 46. At erasing station 37, heat or solvent vaporis applied to smooth out the surface of the thermoplastic and erase anyimages on it that may remain from previous use. This erasing station mayalso suitably include cooling or drying means so that the thermoplasticlayer will be more highly insulating when advanced over photoconductiveweb 35. The plastic support 46 carrying thermoplastic coating 45 istransported along with the movement of the photoconductive insulatinglayer under pressure roller 53. Pressure roller 53 is a conductiveroller with or without an insulating surface layer and having anelectrical connection to reference potential. The electrical referencepermits the roller to apply electrostatic pressure as well as mechanicalpressure to assure a uniform contact between member 46 and web 35. Thelayers are then transported together past the exposure station 40 whichsuitably employs a conventional moving slit exposure means operating insynchronization with the movement of the layers. The exposure stationprojects a pattern of light and shadow through the thermoplastic and itssupport onto the photoconductive insulating layer 35 in accordance withan image subject 47. The latent electrostatic image thus formed appearsas voltage gradients on the surface of the thermoplastic insulatinglayer. The combined layers then pass through the second charging station41 where residual conductivity in the previously illuminated areas ofthe photoconductive layer permits enhanced variations in the chargedensity produced by the voltage sensitive charging device. After thesecond charging, a development station 42 using heat or a solvent vapordevelops the charge density variations on the thermoplastic layer. As inthe case of erasure station 37, development station 42 suitably includescooling or drying means to harden or fix" the thermoplastic layer sothat the deformation image will remain after removal of theelectrostatic image-forming field. The thermoplastic layer along withits support layer have been separated from the photoconductiveinsulating layer and utilized as by a Schlieren optical system forprojection of the image. When the deformable layer is not permanentlybonded to the xerographic member, as in FIG. 11, it is preferred to wetthe surface of the xerographic member before applying the plastic layer.Such wetting helps to eliminate air bubbles and may be added in awashing process that reduces dust or lint buildup on the xerographicplate. Silicone oil such as type DC-200-20CS (Dow Corning), other lightoil or any electrically insulating low viscosity liquid that does notchemically react with the xerographic plate or the plastic layer can beused. FIG. 11 shows bath 50 for applying a liquid film to xerographicweb plate 35.

The present invention has a particular advantage in high resolutionreproduction for high density image storage and the like. Resolutionsgreater than line pairs per millimeter have been obtained. For optimumresolution, certain materisis and processes are preferred. Thephotoconductive material, itself, is preferably selected to have asmooth homogenous surface when coated on a substrate. Suitablephotoconductive coatings are vacuum evaporated vitreous selenium ororganic photoconductors dissolved ina solvent with an organic resinmaterial. The organic solution provides a smooth homogenous coating byspray, whirl or dip coating procedures. Organic photoconductors for thispurpose include 2.5 bis (4' diethyl aminophenyl) 1,3,4 oxadiazole;2.$-bis-(P-aminophenyl)- 1,3,4-triazoles and other 1,3,4 oxadiazole and1,3,4-triazole compounds. One commercially available example in Kalle Toi920, available from Kalle and Co., Wiesbaden-Biebrich, Germany.

The thickness of the layers is a significant factor in high resolutionembodiments. The thickness of the photoconductive layer is not ascritical as the thickness of the overcoatings, but with vitreousselenium the best resolutions have been obtained with a vitreousselenium layer of about 50 microns. Layers from about 20 to 80 micronsof vitreous selenium also produced good results. With other homogenousphotoconductive layers such as organic photoconductive layers, highresolutions have been obtained with layers as thin as about threemicrons.

Of greater significance for high resolution considerations is thethickness of material between the photoconductive surface and thedeformable surface. Empirically it has been found that the maximumresolution that can be obtained is generally limited by the thickness ofsuch material in accordance with the relationship R -k/4d where Rrepresents the resolution in line pairs per millimeter, K is thedielectric constant of the material and d is the thickness inmillimeter. Thus, it has beenfound for resolutions of better than l-linepairs per mm., the thickness of material between the photoconductivesurface and the deformable surface must be less than ten microns thickassuming a dielectric constant of about 4. With the thickness of aninterlayer added to the thickness of the deformable thermoplasticbetween the photoconductive surface and the deformable surface, thedielectric constant must be adjusted accordingly.

While the present invention has been described as carried out inspecific embodiment thereof, there is no desire to be limited thereby,but it is intended to cover the invention broadly within the spirit andscope of the appended claims.

What is claimed is:

l. A method of image reproduction comprising:

a. applying a thin film of electrically insulating low viscosity oil tothe photoconductive surface of a xerographic plate,

b. covering the oiled surface of said plate with a double electricallyinsulatingplastic overlay in which the layer nonadjacent thephotoconductive surface is a thermoplastic having a lower softeningtemperature than the layer adjacent the photoconductive surface,

c. electrostatically charging the surface of the thermoplastic layer,

d. exposing said plate to an image pattern to be reproduced,

e. electrostatically charging the surface of the thermoplastic layer asecond time,

f. softening the thermoplastic layer until it deforms in correspondenceto the image pattern,

g. hardening said thermoplastic layer, and

h. peeling said plastic overlay from said plate.

2. A method of image reproduction comprising:

a. coating the photoconductive surface of a xerographic plate with adouble electrically insulating plastic overlay in which the layernonadjacent the photoconductive surface is a thermoplastic having asoftening temperature layer a second time, e. softening thethermoplastic layer until It deforms in correspondence to the imagepattern, and

f. stripping the double plastic overlay from said photoconductivesurface.

3. A method image reproduction according'to claim 2 in which saidsoftening is produced by application of a solvent vapor;

4. A method of deformation printing comprising:

a. coating a xerographic plate having a transparent supporting substratewith a dimensionally stable deeply colored plastic layer between a fewmicrons and 1 mil. thick,

b. permanently coating said colored plastic with a transparent plasticlayer having a melting temperature between 40 C. and C. and a bulkresistivity at 30 C. of at least 10" ohm-cm.,

c. applying an electrostatic charge to the surface of said transparentplastic layer,

d. projecting an image pattern of light and shadow through saidsubstrate to expose said xerographic plate,

e. softening said second plastic layer with infrared radiation until itdeforms in accordance with the image pattern,

f. cooling said transparent plastic layer, and

g. separating said colored plastic layer supporting said transparentplastic layer from said plate.

5. A process for forming a relief pattern by electrostatic deformationof a thermoplastic surface comprising:

a. coating a photoconductive layer supported on a conductive substratewith a double plastic overlay in which the layer nonadjacent thephotoconductive surface is a thermoplastic layer having a meltingtemperature of about 40 C. to 80C.,

b. electrostatically charging the surface of said thermoplastic layerwith respect to said conductive substrate,

c. simultaneously with said charging, heating said thermoplastic layerto soften it to a viscosity of about l0 to 10 poises,

d. while charging and softening said thermoplastic layer, ex-

posing said photoconductive layer to an image pattern so that saidthermoplastic layer deforms in accordance with said image pattern,

e. cooling said thermoplastic layer until it hardens, and

f. stripping said double plastic overlay from said photoconductivesurface.

6. A method of image reproduction comprising:

a. applying a transparent plastic layer to a xerographic plate,

b. applying a colored thermoplastic layer having a melting temperatureof about 40 to 80' C. to said plastic layer,

c. applying an electrostatic charge to the surface of said thermoplasticlayer while simultaneously softening said thermoplastic layer andexposingsaid xerographic plate to an image pattern of light and shadowto be reproduced,

d. hardening said thermoplastic layer, and

e. stripping said transparent plastic layer carrying said coloredthermoplastic layer from the surface of said xerographic plate to obtainan image reproduction of relatively transparent and colored areas.

2. A method of image reproduction comprising: a. coating thephotoconductive surface of a xerographic plate with a doubleelectrically insulating plastic overlay in which the layer nonadjacentthe photoconductive surface is a thermoplastic having a softeningtemperature between about 40* and 80* centigrade, and the layer adjacentthe photoconductive surface is a dimensionally stable plastic between afew microns and one mil thick, b. electrostatically charging the surfaceof the thermoplastic layer, c. exposing said plate to an image patternto be reproduced, d. electrostatically charging the surface of thethermoplastic layer a second time, e. softening the thermoplastic layeruntil it deforms in correspondence to the image pattern, and f.stripping the double plastic overlay from said photoconductive surface.3. A method image reproduction according to claim 2 in which saidsoftening is produced by application of a solvent vapor.
 4. A method ofdeformation printing comprIsing: a. coating a xerographic plate having atransparent supporting substrate with a dimensionally stable deeplycolored plastic layer between a few microns and 1 mil. thick, b.permanently coating said colored plastic with a transparent plasticlayer having a melting temperature between 40* C. and 80* C. and a bulkresistivity at 30* C. of at least 1013 ohm-cm., c. applying anelectrostatic charge to the surface of said transparent plastic layer,d. projecting an image pattern of light and shadow through saidsubstrate to expose said xerographic plate, e. softening said secondplastic layer with infrared radiation until it deforms in accordancewith the image pattern, f. cooling said transparent plastic layer, andg. separating said colored plastic layer supporting said transparentplastic layer from said plate.
 5. A process for forming a relief patternby electrostatic deformation of a thermoplastic surface comprising: a.coating a photoconductive layer supported on a conductive substrate witha double plastic overlay in which the layer nonadjacent thephotoconductive surface is a thermoplastic layer having a meltingtemperature of about 40* C. to 80* C., b. electrostatically charging thesurface of said thermoplastic layer with respect to said conductivesubstrate, c. simultaneously with said charging, heating saidthermoplastic layer to soften it to a viscosity of about 104 to 106poises, d. while charging and softening said thermoplastic layer,exposing said photoconductive layer to an image pattern so that saidthermoplastic layer deforms in accordance with said image pattern, e.cooling said thermoplastic layer until it hardens, and f. stripping saiddouble plastic overlay from said photoconductive surface.
 6. A method ofimage reproduction comprising: a. applying a transparent plastic layerto a xerographic plate, b. applying a colored thermoplastic layer havinga melting temperature of about 40* to 80* C. to said plastic layer, c.applying an electrostatic charge to the surface of said thermoplasticlayer while simultaneously softening said thermoplastic layer andexposing said xerographic plate to an image pattern of light and shadowto be reproduced, d. hardening said thermoplastic layer, and e.stripping said transparent plastic layer carrying said coloredthermoplastic layer from the surface of said xerographic plate to obtainan image reproduction of relatively transparent and colored areas.